This invention relates to an injection timing control device for distributor-type fuel injection pumps for use in internal combustion engines such as diesel engines, and more particularly to a device of this kind which is adapted to control the fuel injection timing in response to the rotational speed of the engine.
In order to meet requirements imposed by legal regulations concerning exhaust emissions as well as demand for reducing combustion noise during low rotational speed operation, etc. of diesel engines, it has recently been required that injection timing devices for distributor type fuel injection pumps should be designed to vary the fuel injection timing characteristic with respect to the rotational speed of the pump between a lower rotational speed range and a higher rotational speed range.
An injection timing control device of this kind, which is designed to vary the fuel injection timing characteristic as required above, has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 58-5437. This proposed device includes a roller holder, a cylinder forming the housing of the device, and a timer piston slidably received within the cylinder to move the roller holder, the piston defining at opposite ends thereof a hydraulic pressure chamber and a timer spring chamber within the cylinder. The timer piston is displaced by fuel pressure introduced into the hydraulic pressure chamber and variable in proportion to the rotational speed of the engine, thereby varying the circumferential position of the roller holder and hence the fuel injection timing.
According to the proposed device, the timer piston has an end thereof formed with a stepped surface having one or more steps. The cylinder also has an opposed end thereof formed with a stepped surface corresponding in shape to, and adapted to mate with the stepped surface of the timer piston. With such arrangement, when the rotational speed of the engine is in a low range below a predetermined value, the stepped surface of the timer piston is kept mated with the stepped surface of the cylinder, whereby the pressure receiving area of the end of the timer piston is relatively small. On the other hand, when the rotational speed is in a high range above the predetermined value, the timer piston is displaced away from the cylinder to have the stepped surfaces disengaged from each other, whereby the above pressure receiving area increases.
However, in the proposed device, it is required to machine with close tolerances not only the opposed inner and outer peripheral surfaces of the timer cylinder and the piston but also the stepped surfaces of the timer piston and the cylinder, in order to improve the oiltightness and slidability therebetween. Thus, the proposed device had the disadvantage that much labor and time is required to machine two places of the injection timing device with sufficient accuracy.